Hydration and Heat Management

Helpful tips to avoid a summer training disaster

This month, we turn our attention to running through mid-summer heat. Training and racing in the summer can be a true pleasure, but heat and humidity can negatively impact your performance and even be dangerous, particularly if you are not heat acclimatized or the weather is extreme. Even in relatively moderate summer conditions of 80 degrees (or over 70 degrees with high humidity), it is easy to become progressively dehydrated. During a hard workout or race, when your metabolic rate and heat production are high, your core temperature can increase to a hazardous level.

When you run in the heat, your body must cope with both the heat of the environment and the heat produced by your muscles (over two-thirds of the energy produced by your muscles is lost as heat). One of the body’s responses is to send more blood to your skin to enhance cooling, leaving less oxygen-rich blood available for your muscles. Your body also increases sweating to remove heat from your body through evaporative cooling, which makes you progressively more dehydrated. As you become dehydrated your blood volume decreases, so even less blood is available to go to your working muscles, and your heart pumps less blood per beat.

Running on a day that is both hot and humid is more dangerous because sweat rolls off your skin onto the ground rather than cooling you off through evaporation, due to the high moisture content of the air. You still sweat, but the sweat does not have the desired cooling effect, so heat builds up in your body and your core temperature increases. This is particularly true during races, when competitive urges make many runners ignore their body’s warning signals. Races of 5K to 10K in extreme heat and humidity can pose the greatest danger of overheating because your running intensity (and therefore heat production) is so high that your body cannot eliminate heat quickly enough to prevent a steady increase in core temperature. In those conditions, the best plan is to reduce your pace from the start.

With less blood available to the working muscles and the cardiovascular system under stress, running performance is compromised. Research suggests that performance is reduced by about two to three percent for each one percent loss in body weight due to dehydration. If you lose more than about five to six percent of body weight, the risk of heat-related illness increases appreciably. The risk of heat-related illness is also increased by some prescription and over-the-counter drugs, which increase metabolism or decrease sweat rate or blood flow to the skin.

Preventing dehydration

The first step to preventing dehydration is to ensure you are well-hydrated before workouts and races. Your body’s thirst mechanism is a good, but imperfect, indicator of dehydration. Drinking until your urine is lightly colored provides another indication that you are well-hydrated. To top off the tank, the American College of Sports Medicine recommends drinking about a pint of fluid approximately two hours before exercise to help ensure adequate hydration and to allow time to excrete excess water. Avoid beverages containing alcohol and caffeine, as they have a diuretic effect.

How much fluid you lose during running depends primarily on the heat and humidity, and how far you are running. If you lose two percent or less of your body weight during a run, then you can simply rehydrate after you finish, but for longer runs, or in more extreme conditions, you should replace fluids during the run.

The amount of fluid that you should drink during the run depends on how much fluid you have lost and how quickly fluid empties from your stomach. If you drink more than can empty from your stomach, the excess fluid will just slosh around uncomfortably. During running, the stomach can typically empty about five to seven ounces of fluid every 15 minutes (this varies between runners). During a race or hard training run, it is difficult to drink more than three to four ounces at an aid station without stopping.

Running on a hot day, you may sweat out three to four pints of fluid per hour. Since you can only drink one to two pints per hour, you are going to finish somewhat dehydrated. To calculate how much to drink after running, weigh yourself before and afterwards, and drink enough over several hours to bring your weight back to its pre-run level. Because some of the fluid you drink will be lost as urine, you will typically need to drink about one to one-and-a-half times the amount of fluid that you lost.

Remaining dehydrated after running slows down recovery because blood and other body fluids help to remove waste products and bring nutrients to tissues for repair. Quickly replacing lost fluids after running, therefore, will help you to recover as quickly as possible. During the peak of summer, you need a strategy not just for preventing dehydration from each run, but also for preventing cumulative dehydration over many days of hot weather running.

Avoiding hyponatremia

Hyponatremia is a decreased concentration of sodium in the blood, which can lead to brain swelling, seizures and death. Hyponatremia can occur from drinking excessive amounts of water (or other fluids with low sodium levels) before and during long-duration exercise. Runners who do not drink excessively or who use sports drinks or other sodium-containing drinks before and during running have a very low risk of hyponatremia.

The American College of Sports Medicine advises that, "inclusion of sodium (0.5-0.7 grams per liter of fluid) in the rehydration solution ingested during exercise lasting longer than one hour is recommended since it may be advantageous in promoting fluid retention, and possibly preventing hyponatremia in certain individuals who drink excessive quantities of fluid."

Training adjustments

To maintain the quality of your training when the summer heat is intense, try running early in the morning when the temperature is lower. On days when you can predict that you will not be able to maintain your normal pace, face reality and adjust your pace and distance accordingly. When the environmental conditions are severe, you will get a better workout in safer conditions (and will recover more quickly) by running on a treadmill in an air conditioned gym. If you do not have access to a treadmill, do another type of indoor workout or take a day off when the heat (or pollution level) is dangerously high.

Precooling before races

Precooling, which consists of moderately reducing your body temperature before hot weather exercise, is an effective strategy to improve performance for hot weather races. During your warm-up on a hot day, the amount of heat stored in your body increases, and your core temperature starts to rise.

The most convenient way to precool is to wear a cooling vest, which has pockets for ice or gel packs. You simply put on the cooling vest when you start your warm-up and wear it until shortly before the start of the race. This will delay heat accumulation and reduce sweating during your warm-up.